Structural Analysis of the Utica Group in Four Un-Oriented Cores from the Mohawk Valley, New York State: Implications for Tectonics, Burial History, Timing of Vein Formation and Hydrocarbon Migration
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I examined 518.8 m of Utica Group in four un-oriented cores from the Mohawk Valley in New York State and collected fracture data at eight field sites. In cores 74-NY-5, 74-NY-9, 74-NY-10, and 75-NY-11, I recorded 267 fractures and veins, seven normal faults, five reverse faults, six oblique- or strike-slip faults, one instance of bedding-parallel slip, and four dip-slip faults with unresolved motion. I combined data from the cores I examined with three previously examined cores, and existing fracture data from 46 outcrops, with the goal of better understanding the structural history of the Utica Group in the Mohawk Valley. Fracture characteristics from the four cores I examined are consistent with previous structural analyses: few veins or fractures occur in the Utica Group, most features in the cores are vein-filled fractures, most fractures are sub-vertical to steeply dipping, and clusters of higher fracture frequency occur in the cores separated by intervals of few to no fractures or veins. The definitions of four structural domains have been updated based on kinematic indicators from core and outcrop. Domain A is defined by an oldest NNE-striking fracture set, a younger NW-striking set, and dip-slip kinematic indicators. Domain B is defined by an oldest ENE-striking fracture set and strike- and oblique-slip kinematic indicators in outcrop and in cores 74-NY-9 and 74-NY-12. Domain C is defined by the absence of ENE-striking fractures, and Domain D by an oldest E-striking fracture set. The timing of vein formation in the Mohawk Valley is a complex problem, but a new subsidence curve (Macdonald et al., 2017) places the Utica Group deep enough to sustain fracturing during the Ordovician. The same subsidence curve shows the Utica Group entering the oil window in the Ordovician, much earlier than previously thought possible for regional maturation. However, two other subsidence curves show the Utica Group entering the oil window in the Silurian and Devonian, with main gas generation not occurring until the late Carboniferous. The number of orogenies that affected the area suggest that the veins could record different hydrothermal pulses related to those orogenies.